What 393.43(a) means in plain language
This regulation targets the relay valve and emergency valve in your braking system. Both components are responsible for controlling air pressure delivery to your brakes and ensuring the trailer brakes apply automatically if air supply is lost. When either of those valves is defective or not functioning as designed, the vehicle is in violation of 393.43(a).
The relay valve is what allows your brakes to respond quickly by using air pressure from a local reservoir rather than routing everything through a long line from the tractor. The emergency valve is your fail-safe — it's supposed to lock up the trailer brakes if the air connection between the tractor and trailer fails. When these components fail, your ability to stop the vehicle in an emergency is directly compromised.
This is not a paperwork issue or a lights-out citation. It goes to the heart of whether your braking system can actually perform when you need it most. Inspectors know this, which is why the enforcement data tells a very different story than most other Vehicle Maintenance codes.
What our enforcement data actually shows
Across 13 million inspections in our database, 393.43(a) has generated 4,045 all-time citations — placing it at #382 of 3,036 FMCSR codes by total citation volume. That's a mid-tier enforcement frequency, but the out-of-service rate tells the real story.
Our inspection records show that 3,815 of those 4,045 citations resulted in the vehicle being placed out of service — a 94.3% OOS rate. To put that in context, the all-FMCSR average OOS rate across all codes is 31.4%. A 393.43(a) citation places your vehicle out of service at nearly three times the system-wide average rate. Even though the code is technically marked OOS-eligible as "no" in the federal framework, inspectors exercising their discretion under companion brake-system standards are pulling these vehicles at an extraordinary clip.
Perhaps just as notable: our data shows zero citations in the last 90 days and zero citations in the last 12 months. This suggests that 393.43(a) as a standalone cited code has become rare in current enforcement cycles — but when it does appear, it almost always results in a vehicle being parked on the spot.
The CSA severity weight for this violation is 7, which is a significant score that will follow your safety record and affect your carrier's BASIC scores for 24 months from the inspection date.
Who gets cited most
Looking at vehicle makes, our inspection records show that FRHT-coded vehicles account for 401 citations — the highest of any make in this dataset. Kenworth (KW) follows with 335 citations, and FREIGHTLIN-coded units account for 278 citations. PTRB and Volvo-branded trucks also appear frequently in the data. If you're operating one of these platforms, particularly older air-brake equipped tractors, the relay and emergency valve system deserves extra scrutiny during every pre-trip.
On the carrier side, our data shows fleets such as EVANS DELIVERY COMPANY INC (USDOT 38111) with 16 citations and LUCA AUTOTRANSPORTES SA DE CV (USDOT 3210458) with 9 citations appearing at the top of the all-time list. XPO LOGISTICS FREIGHT INC (USDOT 241829) and KAREN YISELL PAZ SALINAS (USDOT 3536809) each show 8 citations in our records. These are simply the carriers where this violation has appeared most frequently across all inspection events in our database — the data does not imply any pattern of negligence.
How severe is this compared to similar codes
Within the Vehicle Maintenance category, 393.43(a) is a low-volume but extremely high-consequence code. Compare it to a few peers:
393.9(a) — Inoperable required lamps has generated 660,737 citations in our database, but carries only a 15.4% OOS rate. Inspectors see inoperable lamps constantly and let most drivers roll with a citation. A relay or emergency valve failure is treated with far less leniency.
396.3(a)(1) — Inspection, repair, and maintenance (general) has 236,919 citations and a 45.3% OOS rate — high in absolute terms, but still less than half the OOS rate you're looking at with 393.43(a).
393.47E — Slack adjuster defective shows 180,363 citations in our records with a 0.0% OOS rate at the code level. That contrast is striking: a defective slack adjuster in our data almost never triggers an OOS order on its own, while a defective relay or emergency valve does so 94.3% of the time.
The pattern across these peer codes is clear: brake-system components that affect dynamic stopping power are treated by enforcement with a severity that citation counts alone don't capture.
How to avoid it
The 94.3% OOS rate means you cannot afford to discover this problem at a weigh station or port of entry. Every action below can be taken before you pull out of the yard:
- Drain your air tanks completely at the start of every pre-trip. As tanks refill, listen for hissing or pressure bleed-down around the relay valve area on the axles and the emergency valve coupling between the tractor and trailer. Any audible leak warrants investigation before departure.
- Test the trailer emergency valve function. With the trailer connected and system fully charged, disconnect the glad-hand supply line and confirm the trailer brakes apply. If they don't lock up immediately, the emergency valve is not working as required.
- Inspect relay valves visually on all axles. On FRHT, KW, and Freightliner platforms — the three most frequently cited makes in our data — the relay valves are typically mounted on or near the axle housing. Look for corrosion, cracked diaphragms, or oil contamination around the valve body.
- Watch for sluggish brake response during your brake test. A relay valve that is partially clogged or sticking will cause delayed or uneven brake application. During your air-brake check, confirm all brakes apply and release evenly.
- Log any brake-response anomalies in your DVIR immediately. A written Driver Vehicle Inspection Report entry creates a repair obligation for your carrier before the next dispatch — which is exactly where this kind of defect should be caught, not on the shoulder of an interstate.